This invention relates to copolyestercarbonates, and more particularly copolyestercarbonates useful for the fabrication of optical disks.
Optical data recording media, including optical disks as exemplified by compact audio disks and CD-ROM disks used in computers, have become a major means of storing data and making it available. The data on an optical disk are read by a plane polarized laser beam, associated with a similar reference beam polarized in a perpendicular direction. For accurate reading, it is necessary to minimize differences in phase retardation of these laser beams upon passage through the disk.
In the further development of optical disks, particularly read-write disks and disks capable of storing larger amounts of data, various physical factors become important. One such factor which is closely related to the storage capability of the disk, is its birefringence, i.e., the difference between indices of refraction of light polarized in two directions perpendicular to each other. Birefringence has several components, caused by such factors as molecular structure of the raw material from which the disk is fabricated and degree of molecular orientation therein. "Orientational" or intrinsic birefringence of a polymer, ideally measured after perfect orientation of all polymer molecules by stretching, is a function solely of molecular structure.
It is possible to calculate the theoretical orientational birefringences of various polymers by considering the birefringence values of the monomers used to synthesize the polymer and their volume proportions in the polymer molecule. Thus, by balancing the birefringences of the monomers it is possible to prepare a polymer which theoretically has an orientational birefringence value of 0.
By far the most common polymers employed in optical disks are polycarbonates and copolyestercarbonates. They are peculiarly suited for this purpose by reason of their transparency and favorable physical properties. However, the bisphenol A polycarbonates, which are the principal ones currently being produced, are characterized by very high birefringence.
In U.S. Pat. No. 4,950,731, a series of polycarbonates having very low orientational birefringence is described. Said polycarbonates are typically copolycarbonates comprising structural units derived from 2,2-bis(4-hydroxyphenyl)propane, also known as "bisphenol A", and 6,6'-hydroxy-3,3,3',3'-tetramethyl-1,1'-spiro(bis)indane, hereinafter designated "SBI". These copolycarbonates are suitable in many ways for the fabrication of optical disks. However, there is room for improvement in such areas as hydrolytic stability, processability and ductility.
In particular, the glass transition temperatures of many SBI-bisphenol A copolycarbonates are above 200.degree. C. and they have relatively high melt viscosities. It is generally very difficult to process materials with such properties. To lower the melt viscosity, one can incorporate structural units derived from "soft block" monomers such as resorcinol, dodecanedioic acid or eugenolpolydimethylsiloxane, but in the case of SBI polycarbonates the proportion of "soft block" units may be so high that the melt stability of the polymer is adversely affected.
It is further found that interfacially prepared polycarbonates sometimes have such disadvantages as poor solubility and swelling behavior of SBI in contact with chlorinated solvents such as methylene chloride. The aforementioned patent describes a particular interfacial method which overcomes these disadvantages to some extent. However, it is cumbersome in that it requires several steps.